1. Field of the Invention
The invention relates to a liquid crystal display (LCD) device and more particularly, to a liquid crystal display device having an improved seal pattern and a method of manufacturing the same.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices are image display devices that utilize the tendency of liquid crystal molecules to align according to an applied voltage. The LCD devices generally include an upper substrate upon which a color filter is formed, a lower substrate upon which a plurality of thin film transistors are formed, and a liquid crystal layer interposed between the upper and lower substrates. The LCD devices display images by controlling changing orientations of the liquid crystal molecules by applying voltage pulses to pixel and common electrodes.
A manufacturing process of the LCD devices includes a thin film transistor array process for forming the lower substrate, an upper substrate forming process, and a liquid crystal cell process. During the thin film transistor array process, a plurality of gate and data lines are formed on a substrate, and a plurality of thin film transistors are formed at crossing portions of the gate and data lines. Then, a pixel electrode is formed in a pixel region of the lower substrate. During the upper substrate forming process, a color filter, a black matrix, and a common electrode are sequentially formed on a substrate. The liquid crystal cell process includes forming an alignment layer, a rubbing process, a cleaning subsequent to the rubbing process, attaching the upper and lower substrates, and injecting the liquid crystal material. The aforementioned liquid crystal cell process will be described more in detail hereinafter with reference to FIG. 1.
FIG. 1 shows a flow chart of a manufacturing process of an LCD device according to the related art.
At step ST1, a first substrate, which has a thin film transistor and a pixel electrode, and a second substrate, which has a color filter layer and a common electrode, are prepared.
Step ST2 forms first and second alignment layers on the pixel electrode and the common electrode, respectively. Step ST2 includes coating a thin polymer film and rubbing the thin polymer film. The thin polymer film may be commonly referred to as an alignment layer. The thin polymer film must be uniformly formed, and the rubbing process must also be performed uniformly on the thin polymer film. The initial orientation of the liquid crystal molecules is determined by the rubbing. The liquid crystal molecules are normally oriented the rubbing of the alignment layer to display a uniform picture. Polyimide may be widely used as a material of the thin polymer film.
Step ST3 forms a seal pattern on either the first substrate or the second substrate. The formation of the seal pattern includes forming a cell gap to allow for injection of the liquid crystal material between the substrates. In addition, the seal pattern prevents the injected liquid crystal material from leaking outside the seal pattern. The seal pattern is commonly fabricated using a screen-printing method or a dispensing method by mixing sealant of thermosetting resin with glass fiber.
During step ST4, spacers are sprayed on one of the first and second substrates to maintain a precise and uniform gap between the first and second substrates. The spacer spray method can be divided into two different types: 1) a wet spray method that involves spraying a mixture of alcohol and spacer material, and 2) a dry spray method that involves spraying spacer material alone.
Here, the seal pattern and the spacers are formed on different substrates. For example, the seal pattern may be formed on the second substrate, which has a relatively even surface, and the spacers may be formed on the first substrate, which functions as a lower substrate of the liquid crystal display device.
During step ST5, the first and second substrates are aligned and then are attached to each other along the seal pattern. The alignment accuracy of the substrates is decided by a margin, and an aligning accuracy of several micrometers is required because light leakage occurs if the substrates are misaligned beyond that margin.
Step ST6 divides the attached substrates into unit cells. The cell cutting process includes a scribing process that forms cutting lines on a surface of the substrate using a diamond pen or a cutting wheel of tungsten carbide, the hardness of which is higher than the hardness of the glass substrate. A breaking process divides the unit cells by using force.
Step ST7 entails injecting a liquid crystal material between two substrates of the unit cells. Each unit cell has an area of several square centimeters and a gap of several micrometers. A vacuum injection method using a pressure difference between the inside and outside of the unit cells is commonly used as an effective injection method.
After finishing the liquid crystal material injection, an injection hole is sealed to prevent leakage of the liquid crystal material. Generally, a ultra violet (UV) curable resin is deposited onto the injection hole by a dispenser, and ultra violet light is then irradiated onto the resin to thereby harden the resin and seal the injection hole. Polarization films are attached on outer surfaces of the unit cell, and a driving circuit is connected to the unit cell using an attachment process.
FIGS. 2A and 2B show processes of forming a seal pattern according to the related art. FIG. 2A shows a seal pattern forming process using a screen-printing method, and FIG. 2B shows another seal pattern forming process using a dispensing method.
FIG. 2A shows a screen 12 that may include a pattern having a specific shape formed thereupon. A squeegee 14 may be used for scrubbing sealing material onto the screen 12. A seal pattern 16 forms on a substrate 10 by scrubbing the sealing material onto the screen 12 using the squeegee 14, for example. The seal pattern may accordingly include formation of a cell gap for subsequent injection of liquid crystal material to thereby prevent the injected liquid crystal material from leaking out of the liquid crystal cell. The seal pattern 16 forms along edges of the substrate 10 and may include at least one injection hole 18 formed at one side thereof.
The seal pattern forming process may include at least two processes. The first process may include formation of the seal pattern 16 on the substrate 10 by scrubbing the sealing material onto the screen 12. Then a second process may include evaporating solvents contained in the sealing material to thereby dry the sealing material.
The thickness of the seal pattern is closely associated with the cell gap of the liquid crystal display device, and it is therefore important to form the seal pattern to have a uniform thickness and height.
The screen-printing method finds wide use due to its convenience, but it is difficult to use on a large substrate. Additionally, the dispensing method applies sealing material onto an entire surface of the screen, followed by scrubbing by the squeegee, and a large amount of sealing material may therefore be consumed.
To solve the above-mentioned problem, a dispensing method that selectively forms the seal pattern only at a desired region, has been gradually adopted. FIG. 2B shows an apparatus for the dispensing method that may include a dispenser 24, a table 20, and a substrate 22 placed on the table 20, wherein the dispensing method may include a syringe for dispensing the sealing material. For example, the seal pattern 26 may be formed by filling the sealing material into the dispenser 24, and then dispensing the sealing material onto the substrate 22 by applying pressure to the syringe while simultaneously moving the dispenser 24 or the table 20. Accordingly, sealing material may be dispensed that has a uniform width and thickness.
As discussed above, the seal pattern may be formed along edges of the substrate, and liquid crystal material is injected into the seal pattern through the injection hole of the seal pattern. Thus the related art seal pattern directly contacts the liquid crystal material.
FIG. 3 shows a cross-sectional view of a liquid crystal display device according to the related art.
FIG. 3 shows first and second substrates 30 and 50 that are spaced apart from and facing each other. A thin film transistor T, which is composed of a gate electrode 32, a semiconductor layer 34, a source electrode 36 and a drain electrode 38, is formed on an inner surface of the first substrate 30. A passivation layer 42 is formed to cover the thin film transistor T. The passivation layer 42 has a drain contact hole 40 exposing a part of the drain electrode 38. A pixel electrode 44 is formed on the passivation layer 42 and connects to the drain electrode 38 through the drain contact hole 40. A first alignment layer 46 is formed to cover the pixel electrode 44.
A black matrix 52 is formed on an inner surface of the second substrate 50 and corresponds to the thin film transistor T of the first substrate 30. A color filter layer 54 is formed on the black matrix 52. A common electrode 56 and a second alignment layer 58 are sequentially formed on the color filter layer 54.
The color filter layer 54, the common electrode 56 and the first and second alignment layers 46 and 58 are formed only in a display region C, which is defined as an area for displaying a picture. Although not shown in FIG. 3, the pixel electrode 44 and the common electrode 56 may extend into a non-display region D so as to electrically connect the substrates 30 and 50.
A seal pattern 60 is formed in the non-display region D, outside the display region C, to attach the substrates 30 and 50. A liquid crystal layer 70 is interposed in the seal pattern 60 between the substrates 30 and 50.
In the related art, a small amount of glass fiber is mixed in to serve as a supporter of the sealant. However, the glass fiber causes bubbles during the blending process of mixing the glass fiber with the sealant. Also, glass fiber is abrasive, and in the dispensing method, the life span of the dispenser shortens due to the glass fiber abrasion.